1. Field of the Invention
The present invention relates generally to serial data communication devices for sending and receiving serial data, and more particularly to a serial data communication device for sending and receiving data through a single signal line, namely half-duplex communication.
2. Description of the Background Art
For communication of serial data, two systems, a full-duplex communication system and a half-duplex communication system, have been generally employed. The full-duplex communication system employs two signal lines: one for transmitting serial data in either way of sending or receiving, and the other for transmitting serial data in the other way. The full-duplex communication system is employed such as for communication of acoustic signals in real time.
However, there is a problem in the full-duplex system that the use of the two signal lines causes a large-scale construction of the signal lines for long distance communication. Furthermore, the requirement of the two signals in this system introduces an increase in the overall cost of communication equipment.
In the half-duplex communication system, two-way communication of sending and receiving data is carried out through a single signal line. That is, the data sending and receiving alternate for each predetermined period of time W. In other words, a single signal line is employed time-divisionally for the two-way communication.
FIG. 4 is a diagram illustrating the operation of communication between two communication units according to the half-duplex communication system of a conventional art. These two communication units are respectively represented as communication units a and b in the following description. Serial data Ai (i=1, 2, 3, . . . ) is sent from the communication unit a and received by the communication unit b. Serial data Bi is sent from the communication unit b and received by the communication unit a. The serial data Ai and Bi normally have the same data length with each other. Further, the serial data Ai and Bi are each transmitted during a period Wa of a predetermined period W (hereinafter referred to as one sending/receiving period), and are not newly provided to a signal line provided between the above described communication units a and b during a period Wb. The period Wb is provided to prevent the state that the communication units a and b both send the serial data simultaneously, namely collision of data. Data sent from one of the communication units reaches the other communication unit after a certain delay time on the signal line provided between the communication units. Therefore, in order to avoid the collision of the data, the data should not be sent from one of the communication units until the data of the communication unit to be sent during one sending/receiving period are all transmitted to the other communication unit. Accordingly, the period Wb is provided during the period from the time when the data sending from one communication unit ends to the time when the data sending from the other communication unit starts. As described above, the directions of transmitting the serial data are switched to each other for each one sending/receiving period W to carry out two-way communication in the half-duplex communication system.
For long distance communication in the conventional half-duplex communication system, the collision of the data occurs when the period Wb is shorter, because of an increased delay time of the serial data on the signal line. In order to avoid the collision of the data, the period Wb should be increased. The period Wa is reduced as the period Wb is increased, so that the serial data of a fixed data length should be transmitted in the reduced period Wa. Therefore, a transmission frequency of the serial data becomes higher, so that the data to be transmitted is susceptible to fluctuation in time such as jitter on a transmission path. Accordingly, it becomes difficult to achieve accurate transmission of the data and stable transmission of the serial data. Normally, the period Wb needs be increased to some extent, assuming that long distance communication is carried out during the period. Thus, data communication through a conventional communication device of the half-duplex system is more affected by fluctuation in time. That is, the conventional communication device of the half-duplex system is less reliable in communication.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.